Future data communications networks, ranging from high performance computers to Fibre-to-the-Home, will rely on cost-effective, power efficient optical transceivers to stem spiralling energy consumption. In IBM's latest supercomputer, the Power 775, a total of 668,000 VCSELs/Fibres were used, each carrying the same wavelength. In next generations, the number of channels required is expected to increase by almost two orders, while energy consumed per bit should reduce by an order of magnitude. This is unimaginable in the framework of VCSELS and multimode fibres, with the physical space consumed and the fibre cost fundamental limits.
Light sources are recognised as a major issue in data communications networks. The current relative success of VCSEL based approaches largely stems from their efficiency. In future, it is anticipated that data communications will have to use dense wavelength division multiplexing (WDM). A problem with VCSELs is that they are largely incompatible with WDM due to poor wavelength control and the difficulty in coupling multiple VCSELs to a single multimode fibre.
There is a number of ways of realising narrow linewidth single mode semiconductor lasers. There are two broad groups, monolithic semiconductor lasers and external cavity lasers. Both use frequency stabilisation to achieve single mode output over a range of operating conditions. The optical gain element is located between a high reflectivity mirror, often the coated back facet of the gain element, and the frequency selective component. Wavelength selectivity is often provided by a Distributed Bragg Reflector (DBR). In a monolithic device, this takes the form of a corrugation of the active layer giving rise to a grating. In an external cavity device, the grating can be realised in fibre, giving rise to a Fibre Grating laser, or in a silicon waveguide.
A difficulty with known lasers is the need to modulate the laser output in order to transmit data. Modulation of the gain is an option, but relaxation oscillations result in a highly chirped output and the bandwidth is limited. External modulators are used in high bandwidth applications. However, integration of these is complex, and discrete components are undesirable on stability grounds. Furthermore, it is difficult to achieve power efficient high speed tuning of components that have a significant capacitances (picofarad). Multiplexing must also be provided. In monolithic systems, this can be provided by III-V semiconductor arrayed waveguide gratings, but the high thermo-optic coefficient of III-Vs provides poor thermal stability. In addition, optical propagation losses and material losses are high. Planar light wave circuits provide high performance multiplexing. However, these are discrete components making assembly complex.